In air conditioner installed in computer server rooms, for example, it is required that the air conditioner be managed at a constant temperature throughout the year even though there are heat sources such as servers, so a cooling operation is required even in a case where the outside air temperature is in a low outside air temperature region equal to or less than 20° C. In the cooling operation in a such a low outside air temperature region, the discharge pressure discharged from the compressor of the air conditioner becomes lower because the outdoor heat exchanger (heat source-side heat exchanger) is caused to function as a condenser, so it becomes difficult to secure the high-low pressure difference that is the difference between the discharge pressure (high pressure) and the suction pressure (low pressure). In order to secure the high-low pressure difference, as described in JP-A No. 2002-39598 for example, it is necessary to precisely control the outdoor fan for the outdoor heat exchanger of the air conditioner in accurate correspondence to the high-low pressure difference.
Consequently, in order to secure the high-low pressure difference in the cooling operation in the low outside air temperature region, detecting the discharge pressure and the suction pressure becomes important. It is common for the suction pressure, which is a low pressure, to be detected by a low-pressure pressure sensor, but methods of detecting the high-pressure discharge pressure include a method where a high-pressure pressure sensor is attached to a high pressure-side pipe of the compressor and a method where the discharge pressure is estimated from a thermistor attached to the outdoor heat exchanger.
If a high-pressure pressure sensor is used, the actual pressure inside the high pressure-side pipe of the compressor can be detected by the high-pressure pressure sensor, so it is possible to control the outdoor fan in accurate correspondence to the high-low pressure difference. However, compared to the case of using a thermistor, using a high-pressure pressure sensor not only becomes disadvantageous to miniaturizing the apparatus because the high-pressure pressure sensor cannot be attached unless a relatively wide space is ensured inside the machine chamber of the outdoor unit, but cost also becomes higher because the product itself is expensive. In addition, stress concentration in a particular place is triggered because the high-pressure pressure sensor becomes stood on the pipe, so a design that improves the strength of the pipe so as to withstand such stress concentration becomes necessary. For that reason, when the demand for space saving and cost reduction is strong, the method where the pressure of the refrigerant inside the high pressure-side pipe is estimated from the temperature of the refrigerant in the outdoor-side heat exchanger detected using a thermistor is employed.